It is well-known to those skilled in the art that whenever a product is heated or cooled, there are a large number of variables that influence the rate at which this happens. The major constraints are usually the thermal diffusivity and thermal effusivity properties of the product being treated. In many instances, there is more than sufficient energy surrounding the product, the constraint is the rate at which the product can absorb the energy and internally distribute it. This in turn e affects product properties and ultimately determines product quality.
The variability of food properties and product quality as a consequence of variation in heat transfer through food is well-known. In many food products, the issues have far-reaching commercial consequences and have received significant attention e.g. coffee brewing (US20100178404) and ice cream and whipped frozen products (US20090191318)
With food and drink products this is principally shown in its organoleptic properties. If the temperature of the energy source is not constrained such as a naked flame or a high temperature oven then the outer surfaces of the foodstuff will rapidly rise well beyond any required processing temperatures and the product properties will change, e.g. browning of meat on a grill or burning of pastry in an oven. In a domestic or food service operation, one is dealing with single entities or small volumes of product and the use of labour to turn, move or otherwise control the rate of heat transfer can be used to control the ultimate properties.
In mass food production, the use of labour or manual intervention to control rate of heat transfer is more problematic and costly. In very high speed production it is impractical.
Consequently, energy transfer usually has an upper or lower limit in the form of temperature control. In a cooking operation, if the upper temperature is limited to say 122° C. (the temperature generally accepted as the minimum needed for effective food sterilization), then no matter how long the product stays in the heating chamber, it will (with a very few exceptions) never exceed 122° C. by more than a few degrees. The same constraints apply to cooling operations.
However, a second variable comes into play, namely dwell time. Excessive dwell time at elevated temperatures can significantly affect product properties as it will lead to protein denaturation, vitamin loss, flavor and texture changes, etc. Therefore, it is essential to maximize the rate of heat transfer for each product. The manner in which that can be achieved is also dependant on a number of variables including product composition, product particle size and size variation, viscosity, container shape, size, composition and dimension, volume, air to solid to solution ratios, headspace, etc. The measures taken for maximizing energy transfer will also vary according the energy transfer method.
For sterilization of food and/or extension of shelf-life, three major technologies have been developed, namely aseptic processing, retorting or continuous heating/cooling technology such as the Stork Hydrostat™.
A simple example, orange juice, will illustrate the different approach to temperature control each has developed.
With aseptic processing, the product is actively passed over or between heat plates so that the product temperature rapidly rises. The flow rate and the temperature differential are controlled so that virtually all the product is subjected to the same temperature and dwell time. The result is that the product is minimally processed and of the highest quality. However, this only happens if the product is homogenous and is effectively free of suspended solids. So processing of pulp free orange juice is effective but the presence of increasing amounts of solids will, because of the temperature of the heating plates cause the solids to denature, the organoleptic properties to change and the quality and uniformity to fall.
The retort process constrains the product in its (usually) final container and attempts to maximize heat transfer through optimizing the stacked product configuration and agitation. This agitation may be agitating the whole retort (U.S. Pat. No. 6,251,337) or the baskets within the retort (EP2177116) or some combination of both—although this latter option has yet to be demonstrated. However the distribution of energy within the retort chamber is not as consistent as with thin-film aseptically processed foodstuffs as, for example, outside products within a stack will heat up faster than ones within the centre of the core.
For those skilled in the art it is known that agitation will move the product within the container, thus improving the heat distribution. This can be achieved in many different ways including rotating food within a static container or pressure vessel, rotating the container, compartmentalized heating and/or cooling for multistage cooking cycles (U.S. Pat. No. 6,071,474).
However, the efficiency of such an operation will be increasingly limited by a whole range of factors such as variation in particle size, product composition and viscosity, container-to-container contact, stacking ratio and format. So, while agitation can improve the energy transfer rate in many products, it will have little effect on others. But the relatively limited size of the batch within the retort chamber makes agitation mechanisms, whether manual, semi-manual or automatic, a practical option.
The large-scale continuous sterilization/pasteurization technologies, such as the Stork Hydrostat™ and the FMC Sterimat™, present a totally different challenge. In these systems, very large volumes of product, often exceeding 1 million units per day, are continuously processed. While there are several variants of this type of technology, the process is essentially the same.
Product to be processed, usually in cans, glass jars or bottles, are loaded into product holders of various configurations. These holders are then passively pulled through a series of chain driven geared drives, through vertical chambers or towers, where each chamber has specific temperature and pressure settings. In the simplest set-up, the first chamber uses hot water to heat up the product to a temperature close to 100° C., the second chamber is a steam chamber where the use of hydrostatic pressure allows the temperature of the steam to rise up to around 125° C. while being constrained within the chamber by the adjacent water columns. The final chambers use cool or cold water to reduce product temperature and stop any further cooking activity.
The usually linear product orientation allows for a more consistent series of processing parameters than retort systems. The method of heating and cooling allows for the effective processing of product with significantly more solid content than an aseptic system. However, the sheer size of the system acts as an enormous energy sink/source and it is not easy to change any system parameters other than dwell time to modify processing conditions. Also, it is virtually impossible to increase throughput capacity because of the essentially constant processing parameters.
The relative slow product throughput speed, typically 6-10 m/m, minimize any product agitation while the typically linear and horizontal product orientation and minimal headspace within the containers further reduce the ability of contents to mix. The result is a process that consistently ensures that product is effectively sterilized but generally the product is of a lower quality than that produced by its aseptic counterparts.
A further issue is the relative inability for such a system to handle and process product in containers constructed of new more cost effective, energy efficient and environmentally friendly materials such as bowls, pouches and plastics such as bottles and yoghurt pots.
A final issue is flexibility in processing. The operating conditions of a retort system, although not optimally performance or cost effective, can be relatively easily modified to allow product to be either sterilized or pasteurized. Currently, continuous sterilizing systems can only be modified with great difficulty and cost. For example, to convert an existing hydrostat based sterilizing system to a pasteurizing system requires the conversion of the chamber/tower from a steam handling unit into a water handling unit.
The embodiments and preferred embodiments of this invention address all of these issues without altering the physical structure or operating principles of the existing system.